1. Field of the Invention
The present invention relates to electroluminescent devices, and more particularly to electroluminescent devices with improved optical out-coupling efficiencies.
2. Description of the Prior Art
Since reports of efficient and practical organic light-emitting devices (OLEDs) in 1987 by Tang and VanSlyke, OLEDs have been subjects of intensive studies and development for displays and lighting applications. A typical OLED has the organic layer(s) sandwiched between one reflective metal electrode (usually cathode) and one transparent indium tin oxide (ITO) electrode (usually anode) on glass substrates (FIG. 1). By adopting efficient emitting materials such the phosphorescence mechanisms, the internal quantum efficiencies of OLEDs can reach nearly 100%. However, in typical OLED structures, the optical out-coupling of OLED internal emission to air is an issue for achieving high external quantum efficiencies. Usually, the ITO and organic layers have higher refractive indices (n; n˜1.8-2.1 for ITO and ˜1.7-1.8 for organic layers in OLEDs) than the typical substrates (e.g., glasses and plastics etc., n˜1.4-1.5) and air (n=1). Thus, due to the significant refractive-index mismatches at air/substrate and substrate/ITO interfaces in typical OLEDs, OLED internal emission usually suffers total internal reflection and hence most of internal radiation is trapped and guided inside the device. In general, internal radiation in OLEDs is coupled into four different modes: radiation modes that are out-coupled to air as useful emission, substrate modes that are trapped and waveguided in the substrate, waveguided modes that are trapped and waveguided in the high-index organic/ITO layers, and surface-plasmon (SP) modes that are guided along the organic/metal interface, as illustrated in FIG. 2. Thus, the out-coupling efficiency of conventional and typical OLED devices is usually only about 20-25%, and there is a great demand in enhancement in external quantum efficiency (EQE) of OLEDs by increasing the light out-coupling, in particular for applications that impose strong requirements on power efficiencies (e.g., lighting and mobile applications etc.).